Sorghum is the second most important cereal-feed grain grown in the United States. Production is economically critical to farms operating in marginal rainfall areas because of sorghum's ability to tolerate drought and heat. Both the livestock and bio-energy industries utilize sorghum as an energy substrate thereby making it a versatile crop.
Worldwide, sorghum is the fifth leading cereal grain. As it is tolerant to both drought and heat, it is easily the most widely grown food grain in the semiarid regions of sub-Sahelian Africa and in the dry central peninsular region of India. As such, sorghum is used in human consumption in most of the driest regions of the world thereby making it a critically important food crop in these locations.
The development of herbicide resistance in plants offers significant production and economic advantages; as such the use of herbicides for controlling weeds or plants in crops has become almost a universal practice. However, application of such herbicides can also result in death or reduced growth of the desired crop plant, making the time and method of herbicide application critical or in some cases unfeasible.
Of particular interest to farmers is the use of herbicides with greater potency, broad weed spectrum effectiveness and rapid soil degradation. Plants, plant tissues and seeds with resistance to these compounds would provide an attractive solution by allowing the herbicides to be used to control weed growth, without risk of damage to the crop. One such class of broad spectrum herbicides are those compounds that inhibit the activity of the acetyl-CoA carboxylase (ACC) enzyme in a plant. Such herbicides are included in the aryloxyphenoxypropionate (FOP) and cyclohexanedione (DIM) chemical families. For example, sorghum is susceptible to many ACC inhibiting herbicides that target monocot species, making the use of these herbicides to control grassy weeds almost impossible.
Certain weed grass species have been found that display altered sensitivity to FOP and DIM herbicides. One grass species, black grass (A. myosuroides [Huds.]), is a major grass weed in Europe. Several mutations have been found in the genome of some black grass plants that confer resistance to some, but not all, FOP and DIM herbicides (Délve, et al., 2005, Plant Phys. 137:794-806; Délye, et al., 2002, Theor. Appl. Genet. 104:1114-1120). Similar findings were found in mutant grass weeds such as annual ryegrass (L. rigidum [Gaud.]; Délye, et al., 2002, Pest Manag. Sci. 58:474-478), green foxtail (S. viridis [L. Beauv.]; Zhang and Devine, 2000, Weed Sci. Soc. Am. 40:33; Délye, et al., 2002, Planta 214:421-427) and wild oat (A. fatua [L.]; Christoffers et al., 2002, Genome 45:1049-1056). One herbicide resistant maize hybrid (DK592 from Dekalb) has a similar mutation in the ACC enzyme as that found in grass weeds (Zagnitko et al., 2001, Proc. Natl. Acad. Sci. 98:6617-22).
Due to the importance of sorghum as a crop plant on the world stage, what are needed are sorghum hybrids that are resistant to the inhibitory effects of ACC herbicides, thereby allowing for greater crop yield when these herbicides are used to control grassy weeds.